Group of agents touring the City (7)
I believe the answer is:
species
'group' is the definition.
(species is a kind of group)
'agents touring the city' is the wordplay.
'agents' becomes 'spies' (spy is a kind of agent).
'touring' is an insertion indicator (touring can mean going round).
'the city' becomes 'ec' (the City of London postcode).
'spies' going around 'ec' is 'SPECIES'.
'of' is the link.
(Other definitions for species that I've seen before include "Taxonomic grouping" , "Category" , "Sort, type" , "Taxonomic group whose members can interbreed" , "Kinds or sorts" .)